As imaging devices, typified by digital cameras, can be downsized easily, they are built into various types of portable devices, such as cell phones, PDAs, and the like. Further, they are widely used as cameras of videophones or vehicle cameras for monitoring images of inside and outside of various types of vehicles from the driver seat. The imaging apparatus includes an imaging unit in which an imaging optical system and a solid-state image sensor that photoelectrically converts a subject image formed by the optical system are unitized, in addition to circuits for reading out an imaging signal by driving the solid-state image sensor and obtaining digitized image data by performing various types of signal processing on the imaging signal, and a memory device for recording the image data.
For example, in the case where an imaging device is incorporated into a mobile phone or PDA widely used as personal digital assistants, it is relatively easy to reduce the thickness of the circuits and memory device by various mounting methods, but it is difficult to reduce the thickness of the imaging unit in view of the optical performance thereof.
Ordinary front illuminated CMOS image sensors, currently being used widely, include microlenses embedded in an incident surface of light from a subject in order to improve the aperture efficiency of pixels and lead beams of an image to portions of the photoelectric conversion section of the respective pixels as much as possible. This may improve the aperture efficiency in comparison with the case in which the microlenses are not provided, whereby photoelectric conversion efficiency is also improved.
As the photoelectric conversion section that photoelectrically converts a subject image is located beneath a wiring layer, some of the incident light beams are blocked by the wiring or the like, thereby causing large loss. Further, a light beam having an incident angle greater than a certain angle may leak to an adjacent pixel, as well as being blocked by the wiring or the like, and the light beam can not contribute to the photoelectric conversion of the original pixel and instead incident on the photoelectric conversion layer of the adjacent pixel. That is, larger incident angle may result in significant problems of sensitivity degradation and color mixture.
Consequently, in the front illuminated CMOS image sensor, the allowable range of spread angle of light beam is limited due to the pixel structure in which the photoelectric conversion section is located below the wiring layer as described above. In the front illuminated CMOS image sensor, it is said that the photoelectric conversion efficiency on the pixel basis is highest when light beams are incident on the imaging surface at right angle, which is reduced sharply from 100% at right-angle incident to about 35% at an incident angle of about 20 degrees and to about 5% of the right-angle incident when the incident angle exceeds about 30 degrees due to blockage of most of the light beams by the wiring layer and the like or leakage to an adjacent pixel. Under the circumstances described above, an optical system used for an imaging device is designed such that the incident angle of the principal ray incident on a peripheral portion of the image screen may fall in about 30 degrees or less as described, for example, in Japanese Unexamined Patent Publication No. 2007-122007, U.S. Pat. Nos. 7,633,690, and 7,602,560. In order to reduce the incident angle as described above, it is necessary to lengthen the overall optical length of the imaging lens (lens group) to some extent, and this is an obstacle to the reduction in overall thickness of the imaging device.
Recently, a significant reduction in pixel pitch has been made and results of experimental production of image sensors with a pixel pitch of less than 1 μm are started to be reported. The reduction in pixel area results in larger sensitivity degradation, so that improvement in photoelectric conversion efficiency has been made and currently a so-called back illuminated type is predominant in order to reduce the loss of incident light due to the wiring in the front illuminated image sensor. In the back illuminated system, the photoelectric conversion section is disposed on the light incident side, which is advantageous for improving the photoelectric conversion efficiency. In the back illuminated system, the cross-talk between adjacent pixels is also a big problem in which large incident angle of incident light causes significant color mixture.
As such, use of organic materials is proposed for the photoelectric conversion layer instead of the conventional silicon as described, for example, in U.S. Pat. No. 8,223,234 and U.S. Patent Application Publication No. 20100245638. As organic materials have a larger visible light absorption coefficient in comparison with silicon, the thickness of the photoelectric conversion layer may be reduced in comparison with the silicon photoelectric conversion layer and the reduction in the thickness may reduce the cross-talk between pixels. Whereas a photoelectric conversion layer made of the conventional silicon requires a thickness of about 3 μm, the thickness of a photoelectric conversion layer made of an organic material may be reduced to as thin as about 0.5 μm. The reduction in thickness of the photoelectric conversion layer may result in a thinner image sensor.
Further, U.S. Patent Application Publication No. 20100245638 proposes an image sensor in which the distance between color filters provided above the photoelectric conversion section and the photoelectric conversion section is 3 μm or less and a separation wall is provided between adjacent color filters. The color filters provided with separation walls allow cross-talk between adjacent pixels to be reduced largely.